Rotary wing aircraft such as helicopters have found many applications due to the vertical flight and hovering capabilities of such craft. These capabilities are achieved through the use of rotary wings, e.g., rotor blades having an airfoil cross-section. As used herein, the term "airfoil" refers to shapes capable of generating lift due to airflow thereabout from a leading to a trailing edge. Rotary wing aircraft are thus capable of generating lift even in vertical flight or while hovering because the rotary motion causes airflow about the surfaces of the rotary wings.
A disadvantage of conventional rotary wing aircraft, i.e., helicopters employing a single main rotor blade assembly in their principal lift generating system, is that such aircraft generally employ a heavy and power consuming tail rotor for torque compensation and yaw control. Torque is exerted on conventional rotary wing aircraft due to the rotation of the main rotor blade assembly which would result in rotation of the aircraft body if not counteracted. Typically, this torque is counteracted by use of a tail rotor which generates a torque equal but opposite to that of the main rotor blade assembly. The pitch of the tail rotor blades may also be adjustable to vary the torque generated by the tail rotor thereby providing helicopter yaw control. Thus, in conventional helicopters, a significant amount of power and weight is dedicated to the tail rotor for torque compensation and yaw control.
Another disadvantage of conventional rotary wing aircraft is the inefficiency and complexity of forward flight relative to fixed wing aircraft. In conventional rotary wing aircraft, a forward thrust is provided by angling the main rotor blade assembly relative to vertical so that a component of the force generated by the assembly is directed forward. By contrast, in a fixed wing aircraft, substantially all of the force generated by a propulsion assembly, such as a propeller or a jet engine, may be directed to provide a forward thrust.
In addition, conventional rotary wing aircraft generally employ a cyclical pitch control assembly to compensate for varying relative air speeds experienced by the rotor blades in forward flight. In rotation the rotor blade has an advancing portion, where the blade is rotating into the "wind" resulting from forward movement of the aircraft, and a retreating portion where the blade is rotating away from the wind. The speed of air relative to a rotor blade section and the force generated by the section in forward flight depends in part upon two components: the speed of forward flight and the speed of the rotor blade section due to rotation of the rotor assembly. As can be understood, these components will be generally additive during the advancing portion of a rotation and generally subtractive at the retreating portion. A complicated cyclical pitch assembly is generally employed in conventional rotary wing aircraft to vary the pitch of the rotor blade over a rotation cycle so that a substantially symmetrical lift and thrust distribution results. To facilitate forward flight, the rotor of conventional rotary wing aircraft is therefore complex in design and operation and generally inefficient in comparison with fixed wing aircraft.
Thus, it would be advantageous if the positive attributes of fixed wing and rotary wing aircraft could be combined. Desirably, such a craft would combine the hovering and vertical flight capability of rotary wing aircraft with the efficiency and simplicity of fixed wing aircraft in forward flight. Additionally, such a craft could preferably eliminate the need for a tail rotor to compensate for rotary wing torque thereby enhancing aircraft weight and power efficiency. Finally, further efficiencies would result if such a craft were provided with a fixed wing capable of generating lift in both forward and vertical flight.